In aircraft control system applications, a nut backing off, and subsequent loss of the bolt, is not acceptable. Therefore, it is typical to provide a hole in the bolt shank, near the end of the threads, so that a cotter pin can be inserted through a castellated (slotted) nut in order to prevent the nut from backing off.
Failure to insert the cotter pin during maintenance can create a safety hazard. Therefore, it is known to provide self-retaining bolts. These bolts have an axial, spring-loaded cam-plunger acting on a ball protruding from the shank (much like the renowned Sears' socket wrench). When the plunger is depressed, the ball can move radially within the shank so that the bolt can be inserted through a close tolerance hole, such as the hole in a control rod end fitting. When the plunger is released, the ball protrudes beyond the shank on the other side of the fitting from the bolt head so that the ball prevents the bolt from coming out of the hole if the nut backs off. However, the hollow shank required to accommodate the plunger creates a noninspectable, corrosion-prone bolt. This is especially pertinent in a salt water spray environment.